Showing papers on "Aircraft noise published in 2015"

Birds living near airports advance their dawn chorus and reduce overlap with aircraft noise

Abstract: Anthropogenic noise is a major pollutant for organisms that live in urban areas. City birds modify their songs in ways that can increase their communication potential in spite of noise. However, these changes cannot prevent song masking by the extremely loud noises to which some urban bird populations are exposed. Here, we show that birds near a major airport advance their dawn singing time, thus reducing overlap with periods of intense aircraft noise. This modification was stronger in species whose normal singing time was relatively late, those which overlapped the most with aircraft noise. Although suggestive of a causal relationship, this pattern does not allow us to tell apart the effect of aircraft noise from that of other variables that may correlate with dawn singing time. In order to control for such potentially confounding variables, we replicated the study in several airports at different latitudes in Spain and Germany. The results show that indeed the overlap of song chorus with aircraft noise was the key factor that influenced time advancement. Aircraft traffic time was the main predictor of song advancement: across Europe, those bird populations whose singing time overlapped the most with aircraft traffic were those that advanced their song timing to a higher extent. Our results exemplify how behavioral plasticity may allow the survival of avian populations in areas of high noise pollution. However, such an adaptation likely involves departing from optimal singing times, leading to higher energetic costs and amplifying between-species differences in competitive ability and resilience.

Transportation planning and quiet natural areas preservation: Aircraft overflights noise assessment in a National Park

Abstract: Global transportation growth causes several disproportionate impacts on the environment as, for instance, noise pollution which is related to negative effects on human health but also to quiet natural areas decline and biodiversity loss. Besides, sound is a component of ecosystems severely threatened by transportation noise disturbance which is related to negative effects on ecosystem functions. This study deals with aircraft noise impact on natural environments from a multiple innovative perspective. It complementarily combines: noise modeling, field measurements, soundscape audibility, human perception and spatial pattern tools for assessing the chronic growing outdoor noise pollution of ecosystems at landscape scale. Firstly, noticeable soundscape degradation from aircraft overflights has been found causing severe acoustic fragmentation and disruptions in the quietness of a national park in Spain (European Union). Air traffic caused sound pressure levels to increase by approximately 8 decibels from natural ambient levels. Secondly, spatial pattern tools together with noise mapping have been found to be useful in providing decision support for decisions-making through anthropogenic noise impact assessment on the natural environment. Finally, public opinion did not perceive aircraft noise-disruption as being as relevant as that quantified by technical procedures. Although 82% of visitors agree that anthropogenic noise pollution may negatively impact on conservation.

Investigation of Broadband Shock Noise from a Jet Near a Planar Surface

Abstract: Many current and future aircraft designs rely on the wing or other aircraft surfaces to shield observers on the ground from the engine noise. However, the available data showing how surfaces interact with a jet to shield and/or enhance the jet noise are currently limited. Therefore, far-field noise data and background-oriented schlieren images were acquired for a round jet, operating in the overexpanded, ideally expanded, and underexpanded supersonic flow regimes, near a planar surface to investigate how airframe surfaces might affect the shock-cell structure in the jet plume and the broadband shock noise produced. These data show that broadband shock noise is produced by the relatively weak shocks far downstream of the nozzle exit; consequently, a surface will be effective at reducing broadband shock noise only if it is long enough to shield the noise produced by shocks. Furthermore, the presence of a surface very near the edge of an underexpanded jet increases the shock-cell spacing, pushing the shock c...

Assessment of Noise Level Variations of Aircraft Flyovers Using Acoustic Arrays

Abstract: Variability in noise levels for flyovers of the same aircraft type can be as large as 12 dB, hampering noise assessment around airports The variable atmosphere (affecting the acoustic propagation) and variations in the aircraft emitted noise are considered as the two main contributors to the noise level variability This paper presents two experiments aimed at quantifying these contributions First, the atmospheric contribution was determined with a loudspeaker (100 m height) sending signals to microphones on the ground, indicating a sound level variability of less than 2 dB Second, noise levels from Boeing 737 flyovers (landings) were measured with an acoustic camera The observed noise level variability was 6–8 dB The acoustic camera imaging capabilities eliminated variations due to ground reflections and ambient noise, and identified the turbofan engines as the dominant noise source Assuming the two contributions to be independent statistical processes, with the atmosphere contributing 2 dB maximal

Estimation of airport noise impacts on public health. A case study of Izmir Adnan Menderes Airport

Abstract: The air transport industry is showing rapid growth in line with the call for meeting the requirements of a rising population. Noise mapping is more useful than surveys and measurements to estimate the effects of noise on public health. In this paper, noise levels for the day, evening and night time slices around Izmir Adnan Menderes Airport were calculated by use of the SoundPLAN 7.2 software according to the European Noise Directive, and the “ECAC Doc. 29-Interim” method was applied for the computation of the aircraft noise. Air traffic data of year 2012, technical information about the airport and geographical data including the layers of elevation, residential buildings, auxiliary buildings, hospitals and schools were used as the main inputs for the model developed in the study. The model was found to perform well for the areas closer to the airport. The results of the study suggested that the area at the north side of the airport, where the city center of Izmir is located, is more affected than other areas. The threshold value of 55 dB(A) was found to be exceeded in 0.3% of the land area covered by Izmir City center during the time slice “day”. The results showed that about 2% of the resident population was exposed to noise levels of 55 dB(A) or higher during day-time in Izmir. In addition, it was understood that the number of people who are potentially exposed to high noise levels and threatened by several illness such as hypertension and sleep disturbances is significant in the surrounding area of the airport. It is thought to be important for airport operators to manage the airport capacity based on the flight schedules in order to control the noise exposure level around the airport.

Development of a Rapid Fleet-Level Noise Computation Model

Abstract: Future air transportation demand forecasts suggest that environmental concerns such as noise will be exacerbated beyond their current level. Although detailed airport noise modeling with tools such as the integrated noise model and the Federal Aviation Administration’s Aviation Environmental Design Tool are available, these software require relatively long setup and run times due to the number of inputs available to the user and the general fidelity level of the models. A rapid, flexible, and more simplified method that reduces the input variables to a critical few and can provide results in minutes is desired to evaluate fleet-level metrics with respect to new technologies or forecasted changes in demand. Current lower-fidelity methods only calculate a change in contour area due to changes within the overall fleet composition. These methods cannot account for the shape of the contour. This paper presents a rapid airport noise computation model that leverages the fidelity of detailed models. By performing...

Development of a SMA-Based, Slat-Gap Filler for Airframe Noise Reduction

Abstract: Noise produced by unsteady flow around aircraft structures, termed airframe noise, is an important source of aircraft noise during the approach and landing phases of flight. Conventional leading-edge-slat devices for high lift on typical transport aircraft are a prominent source of airframe noise. Many concepts for slat noise reduction have been investigated. Slat-cove fillers have emerged as an attractive solution, but they maintain the gap flow, leaving some noise production mechanisms unabated, and thus represent a nonoptimal solution. Drooped-leading-edge (DLE) concepts have been proposed as "optimal" because the gap flow is eliminated. The deployed leading edge device is not distinct and separate from the main wing in DLE concepts and the high-lift performance suffers at high angles of attack (alpha) as a consequence. Elusive high-alpha performance and excessive weight penalty have stymied DLE development. The fact that high-lift performance of DLE systems is only affected at high alpha suggests another concept that simultaneously achieves the high-lift of the baseline airfoil and the noise reduction of DLE concepts. The concept involves utilizing a conventional leading-edge slat device and a deformable structure that is deployed from the leading edge of the main wing and closes the gap between the slat and main wing, termed a slat-gap filler (SGF). The deployable structure consists of a portion of the skin of the main wing and it is driven in conjunction with the slat during deployment and retraction. Benchtop models have been developed to assess the feasibility and to study important parameters. Computational models have assisted in the bench-top model design and provided valuable insight in the parameter space as well as the feasibility.

On Physical Aeroacoustics with Some Implications for Low-Noise Aircraft Design and Airport Operations

Abstract: Air traffic is growing at a steady rate of 3% to 5% per year in most regions of the world, implying a doubling every 15–25 years. This requires major advances in aircraft noise reduction at airports, just not to increase the noise exposure due to the larger number of aircraft movements. In fact it can be expected, as a consequence of increased opposition to noise by near airport residents, that the overall noise exposure will have to be reduced, by bans, curfews, fines, and other means and limitations, unless significantly quieter aircraft operations are achieved. The ultimate solution is aircraft operations inaudible outside the airport perimeter, or noise levels below road traffic and other existing local noise sources. These substantial noise reductions cannot come at the expense of a degradation of cruise efficiency, that would affect not just economics and travel time, but would increase fuel consumption and emission of pollutants on a global scale. The paper reviews the: (i) current knowledge of the aircraft noise sources; (ii) the sound propagation in the atmosphere and ground effects that determine the noise annoyance of near-airport residents; (iii) the noise mitigation measures that can be applied to current and future aircraft; (iv) the prospects of evolutionary and novel aircraft designs towards quieter aircraft in the near term and eventually to operations inaudible outside the airport perimeter. The 20 figures and 1 diagram with their legends provide a visual summary of the review.

The relationship between aircraft noise exposure and day-use visitor survey responses in backcountry areas of national parks

Abstract: To evaluate the relationship between aircraft noise exposure and the quality of national park visitor experience, more than 4600 visitor surveys were collected at seven backcountry sites in four U.S. national parks simultaneously with calibrated sound level measurements. Multilevel logistic regression was used to estimate parameters describing the relationship among visitor responses, aircraft noise dose metrics, and mediator variables. For the regression models, survey responses were converted to three dichotomous variables, representing visitors who did or did not experience slightly or more, moderately or more, or very or more annoyance or interference with natural quiet from aircraft noise. Models with the most predictive power included noise dose metrics of sound exposure level, percent time aircraft were audible, and percentage energy due to helicopters and fixed-wing propeller aircraft. These models also included mediator variables: visitor ratings of the "importance of calmness, peace and tranquility," visitor group composition (adults or both adults and children), first visit to the site, previously taken an air tour, and participation in bird-watching or interpretive talks. The results complement and extend previous research conducted in frontcountry areas and will inform evaluations of air tour noise effects on visitors to national parks and remote wilderness sites.

Development of a SMA-Based, Slat-Gap Filler for Airframe Noise Reduction

Abstract: Noise produced by unsteady flow around aircraft structures, termed airframe noise, is an important source of aircraft noise during the approach and landing phases of flight. Conventional leading-edge-slat devices for high lift on typical transport aircraft are a prominent source of airframe noise. Many concepts for slat noise reduction have been investigated. Slat-cove fillers have emerged as an attractive solution, but they maintain the gap flow, leaving some noise production mechanisms unabated, and thus represent a nonoptimal solution. Drooped-leading-edge (DLE) concepts have been proposed as "optimal" because the gap flow is eliminated. The deployed leading edge device is not distinct and separate from the main wing in DLE concepts and the high-lift performance suffers at high angles of attack (alpha) as a consequence. Elusive high-alpha performance and excessive weight penalty have stymied DLE development. The fact that high-lift performance of DLE systems is only affected at high alpha suggests another concept that simultaneously achieves the high-lift of the baseline airfoil and the noise reduction of DLE concepts. The concept involves utilizing a conventional leading-edge slat device and a deformable structure that is deployed from the leading edge of the main wing and closes the gap between the slat and main wing, termed a slat-gap filler (SGF). The deployable structure consists of a portion of the skin of the main wing and it is driven in conjunction with the slat during deployment and retraction. Benchtop models have been developed to assess the feasibility and to study important parameters. Computational models have assisted in the bench-top model design and provided valuable insight in the parameter space as well as the feasibility.

Community response to noise in Vietnam: Exposure-response relationships based on the community tolerance level

Abstract: Social surveys on noise annoyance have been conducted in five different cities in Vietnam. The surveys included both aircraft noise (three airports) and road traffic noise (five cities). The main objective for these studies was to establish dose-response functions that were representative for Vietnam. The results have been compared with results from similar surveys from other regions. Dose-response functions for aircraft noise in Vietnam showing the percentage of highly annoyed people versus the noise level are nearly identical to those presented in the European Noise Directive [European Commission (2002). http://ec.europa.eu/environment/noise/directive.htm]. For road traffic noise, however, the results indicate that people in Vietnam are more tolerant. The noise levels can be increased by 5–10 dB in order to have a response similar to the curve recommended by the European Commission.

Characterization of Low Noise Technologies Applied to a Full Scale Fuselage Mounted Nose Landing Gear

Abstract: The negative impact of aircraft noise includes effects on population’s health, land use planning and economic issues such as building restrictions and operating restrictions for airports. Thus, the reduction of noise generated by aircraft at take-off and approach is an essential consideration in the development of new commercial aircraft. Among the different aircraft noise sources, landing gear noise is one of the most significant during approach. This research presents results from the European Clean Sky funded ALLEGRA project, which investigated a full-scale Nose Landing Gear (NLG) model featuring the belly fuselage, bay cavity and hydraulic dressing. Tests were performed for a variety of wind speeds and yaw angles. In this paper, a characterization of the noise generated by the full-scale Nose Landing Gear (NLG) model is presented and the different techniques used for characterizing acoustic sources on the NLG are described. The landing gear noise source is characterized in terms of OASPL, directivity, source spectra, PNL and PNLT. A comparison between the NLG with and without the application of low noise technology is presented.Copyright © 2015 by ASME

System Noise Assessment of Blended-Wing-Body Aircraft with Open Rotor Propulsion

Abstract: An aircraft system noise study is presented for the hybrid wing–body aircraft concept with open-rotor engines mounted on the upper surface of the airframe. The aircraft chosen for the study is of a size comparable to the Boeing 787 aircraft. It is shown that, for such a hybrid wing–body aircraft, the cumulative effective perceived noise level is about 24 dB below the current aircraft noise regulations of stage 4. Although this makes the design acoustically viable in meeting the regulatory requirements, even with the consideration of more stringent noise regulations in the next decade or so, the design will likely meet stiff competition from aircraft with turbofan engines. The noise levels of the hybrid wing–body design are held up by the inherently high noise levels of the open-rotor engines and the limitation on the shielding benefit due to the practical design constraint on the engine location. Furthermore, it is shown that the hybrid wing–body design has high levels of noise from the main landing gear,...

Assessment of aircraft noise sources variability using an acoustic camera

Abstract: Noise assessment around airports is hampered due to the observed large variability in noise levels for fly-overs of the same aircraft type, which is not considered by the current models. This paper assumes that the noise variability is due to variations in the aircraft emitted noise, neglecting the effect of the variable atmosphere, as previous work showed that its contribution is maximally 2 dB. In order to quantify and investigate the variability of noise levels during aircraft fly-overs, 115 measurements of noise of landing aircraft were taken using a 32 microphone array. The noise levels from Boeing 737 fly-overs were analyzed and the noise level variability was determined to be approximately 16 dB. After determining the engine settings from the spectrograms, it was found that variations on the engine settings explain over 55% of the observed total noise variation. In addition, by performing beamforming on the acoustic data, it was confirmed that airframe noise (from the landing gear and deployed flaps) is dominant for several frequencies, especially for modern aircraft.

Aircraft Noise: The major sources, modelling capabilities, and reduction possibilities

Abstract: In October 2014, the first ”Joint DLR & TU Delft Aviation Noise Workshop” was organized. This publication is the executive summary of this event. Overall, 38 invited participants from industry, academia, and research institutions have discussed the specific topic of this first 3 day workshop, i.e ”Aircraft Noise Reduction at the Source”. Four specific tasks were formulated in order to address the problem, i.e. (1) identification of main aircraft noise sources on-board of a given reference vehicle, (2) assessment of simulation capabilities for noise prediction, (3) identification and assessment of promising noise reduction concepts for the reference vehicle, and (4) integration of these measures on-board of the reference vehicle. The major noise sources on-board of the reference vehicle as identified by the participants could have been reduced significantly if selected measures are installed on-board. These proposed measures promise to reduce the system noise by 8 dB along a take-off and by 10 dB along an approach flight. Yet, the almost 65% reduction in perceived noise as specified by ACARE’s Flight Path 2050 could not be achieved. The most effective measure has been identified as structural shielding of engine noise emission.

The effects of noise on key workplace skills.

Abstract: This study explored the effect on memory and psychomotor performance of wideband noise (simulated in-cabin aircraft noise) at 75 dBA, which is similar to that experienced during the cruise phase of a commercial flight. The results from the tests were compared to the effects of a widely known and common metric on the same skills, namely, blood alcohol concentration (BAC). All 32 participants, half non-native English speakers, completed three different tests (recognition memory, working memory, and reaction time) presented in counterbalanced order, either in the presence of noise, with or without noise attenuation headphones, and without noise but with a BAC of 0.05 or 0.10. Simulated aircraft noise was found to affect recognition memory but not working memory or reaction time. These effects were more pronounced for non-native speakers and reflected performance similar to that for BAC of 0.05 or 0.10.

A Method for Simulation of Rotorcraft Fly-In Noise for Human Response Studies

Abstract: The low frequency content of rotorcraft noise allows it to be heard over great distances. This factor contributes to the disruption of natural quiet in national parks and wilderness areas, and can lead to annoyance in populated areas. Further, it can result in detection at greater distances compared to higher altitude fixed wing aircraft operations. Human response studies conducted in the field are made difficult since test conditions are difficult to control. Specifically, compared to fixed wing aircraft, the source noise itself may significantly vary over time even for nominally steady flight conditions, and the propagation of that noise is more variable due to low altitude meteorological conditions. However, it is possible to create the salient features of rotorcraft fly-in noise in a more controlled laboratory setting through recent advancements made in source noise synthesis, propagation modeling and reproduction. This paper concentrates on the first two of these. In particular, the rotorcraft source noise pressure time history is generated using single blade passage signatures from the main and tail rotors. These may be obtained from either acoustic source noise predictions or back-propagation of ground-based measurements. Propagation effects include atmospheric absorption, spreading loss, Doppler shift, and ground plane reflections.

Variability in the Propagation Phase of CFD-Based Noise Prediction: Summary of Results From Category 8 of the BANC-III Workshop

Abstract: The usage of Computational Fluid Dynamics (CFD) in noise prediction typically has been a two part process: accurately predicting the flow conditions in the near-field and then propagating the noise from the near-field to the observer. Due to the increase in computing power and the cost benefit when weighed against wind tunnel testing, the usage of CFD to estimate the local flow field of complex geometrical structures has become more routine. Recently, the Benchmark problems in Airframe Noise Computation (BANC) workshops have provided a community focus on accurately simulating the local flow field near the body with various CFD approaches. However, to date, little effort has been given into assessing the impact of the propagation phase of noise prediction. This paper includes results from the BANC-III workshop which explores variability in the propagation phase of CFD-based noise prediction. This includes two test cases: an analytical solution of a quadrupole source near a sphere and a computational solution around a nose landing gear. Agreement between three codes was very good for the analytic test case, but CFD-based noise predictions indicate that the propagation phase can introduce 3dB or more of variability in noise predictions.

Acoustic Prediction of Helicopter Unsteady Manoeuvres

Abstract: Real-time estimation techniques of helicopter noise in manoeuvring flight are examined. They consist of quasi-steady aeroacoustic predictions based on noise-sources (given in terms of hemispheric acoustic maps rigidly connected to the rotorcraft) suitably extracted from a database generated through off-line aeroacoustic analysis of rectilinear, steady-state flights. This is of interest for the Clean Sky GRC5 MANOEUVRES project, which aims at developing an in-flight noise monitoring system to make the pilot aware of acoustic disturbance produced. Considering a helicopter in an approach manoeuvre, the accuracy of the quasi-steady acoustic approaches is assessed by comparison with fully unsteady simulations, and the sensitivity of their predictions on the quality of the estimation of the flight parameters considered to extract the suitable instantaneous hemispheric acoustic maps from the database is examined.

Aeroacoustic Imaging Experiments of Airframe Noise in Lined Wall Closed-Section Wind Tunnel

Abstract: Acoustic imaging is an indispensable experimental aid in the design of silent aircraft. In this work, acoustic imaging is performed in a classical closed-section wind tunnel that has been extensively modified using specifically designed liners on the walls of the test section. The details of the associated acoustic modification and the aeroacoustic imaging method are introduced in this paper. The acoustic test performance is carefully evaluated by examining background noise reductions and acoustic beamforming results. The experimental outcomes suggest that the proposed acoustic modification could suppress background noise by 5 dB at 40–80 m/s test flow speeds. The beamforming results clearly identify the dominant airframe noise sources at landing gears and high-lift devices. The experimental results agree fairly well with the preceding computational and empirical predictions. The proposed acoustic imaging technique has been shown to be an effective experimental technique in identifying airframe n...

Numerical Investigation of the Refraction Effects by Jet Flows in Anechoic Wind Tunnels, with Application to NASA/LaRC Quiet Flow Facility

Abstract: In regard to the problem of aircraft noise mitigation, the present study focuses on the refraction effects to be possibly induced by anechoic facility jet flows on the measured acoustic signatures, during typical airframe noise experiments. To this end, numerical simulations are conducted, enabling the characterization of the refraction effects induced by a typical open-jet, anechoic wind tunnel, namely the NASA Langley’s Quiet Flow Facility (QFF). Computations are conducted using two complementary approaches, that is, Computational AeroAcoustics (CAA) relying on the Perturbed Euler Equations (PEE) and Geometrical Acoustics (GA) based on a Ray Tracing (RT) technique. These complementary CAA/PEE and GA/RT simulations allow exploring a wide range of canonical situations, that is, comprising jet flows of various natures (planar vs. round, parallel vs. spreading, etc.) and acoustic sources of diverse characteristics (frequency, location, etc.). The analysis of numerical results highlights the respective effects by the flow features, delivering insights about what are the key parameters (jet characteristics, source-to-jet aspect ratio, etc.) to play a major role in the refraction phenomena by the facility jet flow.

System Noise Prediction of the DGEN 380 Turbofan Engine

Abstract: The DGEN 380 is a small, separate-flow, geared turbofan. Its manufacturer, Price Induction, is promoting it for a small twinjet application in the emerging personal light jet market. Smaller, and producing less thrust than other entries in the industry, Price Induction is seeking to apply the engine to a 4- to 5-place twinjet designed to compete in an area currently dominated by propeller-driven airplanes. NASA is considering purchasing a DGEN 380 turbofan to test new propulsion noise reduction technologies in a relevant engine environment. To explore this possibility, NASA and Price Induction have signed a Space Act Agreement and have agreed to cooperate on engine acoustic testing. Static acoustic measurements of the engine were made by NASA researchers during July, 2014 at the Glenn Research Center. In the event that a DGEN turbofan becomes a NASA noise technology research testbed, it is in the interest of NASA to develop procedures to evaluate engine system noise metrics. This report documents the procedures used to project the DGEN static noise measurements to flight conditions and the prediction of system noise of a notional airplane powered by twin DGEN engines.

Determining an empirical emission model for the auralization of jet aircraft

Abstract: Aircraft noise is a major issue in urban areas and is one of the research topics within the FP7 SONORUS project. Current methods for determining the impact of aircraft noise on annoyance and sleep disturbance are based on energetic quantities neglecting the dynamic character of the sound. To obtain a more complete representation of annoyance, it would be helpful to predict the audible aircraft sound and determine the impact of the aircraft sound on people. In a related project at Empa, sonAIR, recordings were made of aircraft taking off and landing. These recordings were made at several positions and with several microphones simultaneously. Combined with cockpit data, flight path information and an inverse sound propagation model, this gives the possibility to determine the emission as function of aircraft conditions and observer angle. An inverse sound propagation model is used to estimate the emission in the time-domain. The obtained signal corresponds to the immission of a microphone flying along with the aircraft and rotating about it. The time-domain approach allows extracting narrowband information like tones and time-dependent variations like modulations.

Simple Tool for Aircraft Noise-Reduction Route Design

Abstract: The design of arrival and departure routes from an airport has to balance the conflicting requirements of fuel efficiency, airport capacity utilization and community emission and noise considerations. The commonly used tools for aircraft noise assessment are the FAA’s Integrated Noise Model (INM) and Aviation Environmental Design Tool (AEDT). These tools are suitable to generate precise noise contours. However, they are harder to use with other tools for route design optimization involving evaluation of a large number of aircraft trajectories. A simplified aircraft noise computation tool, named AIRNOISE, is developed for preliminary aircraft noise-reduction route design in this paper. AIRNOISE computes aircraft noise based on the same SAE-AIR-1845 procedures used by INM and AEDT. AIRNOISE does not consider components related to terrain and atmosphere adjustments. As a result, it is not only computationally efficient but also flexible to use for customized aircraft profiles. The aircraft noise results are compared with the FAA’s AEDT2b and show that the level of accuracy achieved by AIRNOISE can be used to reduce the number of route design options to a small number from a large pool for subsequent accurate analysis by INM.

Estimation of aircraft angular coordinates using a directional-microphone array—An experimental studya)

Abstract: Ground reflections cause problems when estimating the direction of arrival of aircraft noise. In traditional methods, based on the time differences between the microphones of a compact array, they may cause a significant loss of accuracy in the vertical direction. This study evaluates the use of first-order directional microphones, instead of omnidirectional, with the aim of reducing the amplitude of the reflected sound. Such a modification allows the problem to be treated as in free field conditions. Although further tests are needed for a complete evaluation of the method, the experimental results presented here show that under the particular conditions tested the vertical angle error is reduced ∼10° for both jet and propeller aircraft by selecting an appropriate directivity pattern. It is also shown that the final level of error depends on the vertical angle of arrival of the sound, and that the estimates of the horizontal angle of arrival are not influenced by the directivity pattern of the microphones nor by the reflective properties of the ground.